Heat pipes comprise at least one heat conveying pipe filled with a heat carrier, also referred to as medium. At least one flow channel for the liquid phase of the medium and one flow channel for the vapor phase of the medium are provided in the heat conveying pipe. Heat pipes are also equipped with features for removing bubbles from the liquid flow channel. Furthermore, at least one radiator or heat exchanger is connected to the heat pipe in a heat exchanging contact.
As mentioned, heat pipes for the transport of heat are known, especially from their use in space technology. These heat pipes operate by evaporating the medium at a heat receiving end of the heat pipe and then transporting the vapor to a heat discharging end of the heat pipe where the vapor is condensed again and returned to the evaporating end of the pipe. The medium is conventionally ammonia. As the vapor condenses at the heat discharging end of the pipe, the latent heat stored in the vapor is discharged to the surroundings of the spacecraft while the condensate being formed flows back to the heat receiving evaporating end of the pipe. The transport of the vapor from the evaporating end to the condensing end is a normal compression flow while the flow of the liquid from the condensing end to the evaporating end is a capillary flow. Different radii of curvature along the boundary surface between the liquid and the vapor at the evaporating end of the pipe on the one hand, and at the condensating end of the pipe on the other hand, and the capillary forces caused by these different radii, result in a pressure difference in the direction from the condensating end of the pipe toward the evaporating end of the pipe and this pressure difference maintains the required flow. The resulting flow velocity is established by the equilibrium between the pressure loss due to friction forces and the effective pressure difference of the capillary forces.
Modern high performance heat pipes are capable of transporting heat quantities within the range of about 1 kw over distances between 1 and about 20 m, even at relatively low temperature differences between the evaporating and condensating ends of the pipe.
Comparing these high performance heat pipes with other conventional heat pipes, the higher power of the high performance heat pipes is achieved, due to the fact that the transport of the liquid takes place in channels having differing dimensions. On the one hand, in the evaporating area of the pipe, a plurality of very small channels are provided which extend in the circumferential direction and which have capillary geometries in order to achieve large capillary driving forces. On the other hand, the guidance of the flow in the condenser area and return flow path of the pipe, there are only a few flow channels or even a single flow channel having a relatively large diameter. These few channels or the single channel are also referred to as "artery channels". In this manner friction caused pressure losses are minimized and a substantially larger fluid mass flow is achieved with the same capillary forces as are present in normal heat pipes. As a result of the substantially larger fluid mass flow, a substantially higher heat flow is also achieved.
However, a substantial problem is encountered in the operation of such high performance heat pipes in that the function of these high performance heat pipes is substantially adversely affected or may even be totally interrupted when bubbles are formed of the medium vapor or of gaseous non-condensable contaminations in the artery channels. These contaminations could have been present in the heat pipe already at the time of putting the heat pipe into service or these contaminations could have been generated, for example, by an operational overloading of the heat pipe, such as could occur by an overheating of the evaporator end when a short duration complete drying of the evaporator end of the heat pipe should occur. These bubbles can even interrupt the transport of the heat carrier fluid to the heat take-up or evaporator zone so that the heat take-up zone even dries further and thus the heat pipe becomes inoperative, in other words, ceases to function properly.
In a publication "Heat Pipe Design Handbook", Volume 1, by E & K Engineering, Inc., Towsen, Md., 21204, pages 147 to 153, and especially pages 149 and 152, two heat pipes are described with features for removing bubbles, and thus for avoiding blockage of the fluid flow by these gas bubbles. In one of the conventional heat pipes, the gas bubbles are removed by the arrangement of venting bores in the boundary wall between the artery and the vapor flow channel. In the other conventional construction the bubble removing feature includes a Venturi nozzle which is arranged in the transport channel for the vapor and which simultaneously functions as a jet pump for sucking off any gas bubbles that may be present in the artery.
A disadvantage of having venting bores in the boundary wall between the artery and the vapor channel, is seen in the fact that during the operation of the heat pipe, the pressure in the vapor channel is substantially higher than in the artery. As a result, it is necessary to interrupt the operation of the heat pipe for transferring gas bubbles from the artery into the vapor channel. However, during such interruption of the operation, the venting bores are covered by liquid bridges which block the passage of gas bubbles through these venting bores unless these liquid bridges are first evaporated. As a result, these interruptions of the operation of the heat pipe require a relatively long time duration for the gas bubble removal before the heat pipe can be returned to its normal operation.
The arrangement of a Venturi nozzle in the vapor channel has the following disadvantage. If there happens to be no gas bubble in the suction zone of the nozzle, a small quantity of heat carrier medium tends to collect in the suction pipe of the nozzle and this medium is taken out of the artery. If now a gas bubble appears in fact in front of the suction opening of the Venturi nozzle, it is necessary to first remove the liquid accumulated in the suction pipe before the bubble can be sucked out of the artery. As a result of this procedure, there is a substantial pressure loss in the flow through the suction pipe which correspondingly results in a substantial pressure loss in the Venturi pipe. Stated differently, this Venturi pipe must be constructed to have a relatively substantial reduction in its flow cross-sectional area. This requirement in turn leads to a substantial impairment of the vapor flow due to the pressure loss, whereby the working capacity of the heat pipe is respectively reduced.